1. Field of the Invention
The present invention relates to a valve timing control system for controlling valve operation timing(s) for an intake valve and/or an exhaust valve of an internal combustion engine.
2. Description of Related Art
As the conventional valve timing control system for the internal combustion engine known heretofore, there may be mentioned a control system such as disclosed, for example, in Japanese Patent Publication No. 317504/1997. In this known valve timing control system, such an arrangement is adopted in which one of the intake valve and the exhaust valve which exhibits such valve characteristic which allows the characteristic value of the internal combustion engine to attain speedily a desired or target characteristic value is selected, whereon control is performed such that the amount of a hydraulic fluid supplied to one of actuating mechanisms which is destined for changing the characteristic of the valve selected is increased more than the amount of the hydraulic fluid supplied to the other actuating mechanism.
For better understanding of the concept underlying the present invention, description will first be made in some detail of the conventional valve timing control system for the internal combustion engine known heretofore. FIG. 18 of the accompanying drawings is a block diagram showing schematically and generally a configuration of a conventional valve characteristic (or performance) control system. As can be seen in the figure, this known valve timing control system is comprised of an intake valve M4 for opening/closing an intake pipe M3 for the intake air which pipe leads to a combustion chamber M2 defined within a cylinder of an internal combustion engine M1, an exhaust valve M7 for opening/closing an exhaust pipe M6 provided for discharging an exhaust gas resulting from the combustion of the air-fuel mixture within the combustion chamber M2 within the engine cylinder, first and second actuating mechanisms M5 and M8 adapted to be actuated under hydraulic pressure for changing valve characteristics of the intake valve M4 and the exhaust valve M7, respectively, a hydraulic fluid supply source M10 for supplying a hydraulic fluid such as oil to both the actuating mechanisms M5 and M8 by way of fluid passages M9, respectively, a hydraulic fluid flow regulating means M11 for regulating or adjusting the amount or rate of the hydraulic fluid supplied from the hydraulic fluid supply source M10 to both the actuating mechanisms M5 and M8, respectively, an engine operating state detecting means M12 for detecting the operating states of the internal combustion engine M1, a control means M13 for changing the characteristics or performances of both the intake and exhaust valves, respectively, through regulation by the hydraulic fluid flow regulating means M11 on the basis of the detected engine operation state so that the characteristic value of the internal combustion engine M1 can assume the desired or target characteristic value, wherein the control means M13 is so designed as to select on the basis of the prevailing engine operating state one of the intake valve M4 and the exhaust valve M7 which exhibits such valve characteristic which causes the characteristic value of the internal combustion engine M1 to reach more speedily the target characteristic value, while controlling simultaneously the hydraulic fluid flow regulating means M11 so that the amount of hydraulic fluid supplied to one of the actuating mechanisms (M5 or M8) which is destined for changing the valve characteristic of the selected valve is increased more than the amount of the hydraulic fluid supplied to the other actuating mechanism (M8 or M5).
At this juncture, it should be mentioned that the term "valve characteristic" used herein means at least one of a valve-open timing or a valve-close timing of the intake valve and the exhaust valve. Further, with the phrase "characteristic value of the internal combustion engine", it is contemplated to mean a quantity which changes in response to the changes of the intake valve and the exhaust valve such as, for example, output characteristic, exhaust characteristic, fuel consumption ratio, stability in the idling operation of the engine or the like.
As will be appreciated from the foregoing, in the conventional valve timing control system of the structure described above, such arrangement is adopted in which one of the intake valve and the exhaust valve which exhibits the valve characteristic which allows the characteristic value of the internal combustion engine to reach speedily a desired or target characteristic value is selected, whereon control is performed such that the amount of the hydraulic fluid supplied to one of the actuating mechanisms which is destined for changing the characteristic of the valve selected is increased more than that of the hydraulic fluid supplied to the other actuating mechanism.
The reason why such control as mentioned above is performed may be explained by the fact that in case a plurality of valve timing control systems or mechanisms are actuated with one and the same hydraulic fluid supply source, the oil or hydraulic pressure to be supplied becomes lower (i.e., amount of the hydraulic fluid or oil decreases) when compared with the case a single valve timing control system or mechanism is actuated, which thus incurs lowering or degradation of the responsivity of the valve timing control means or mechanisms. Such being the circumstances, the selected valve timing control means or mechanism is actuated with priority over the other.
Thus, although sufficient responsivity can be ensured for the valve timing control means or mechanism actuated with priority, the other valve timing control means or mechanism(s) will suffer shortage of oil supply, incurring degradation in the responsivity thereof more or less.